The present invention relates in general to integrated circuits and, more particularly, to a circuit and method of controlling mixer linearity in a receive signal path.
Receiver circuits are used in a myriad of wireless communication applications such as cordless telephones, pagers, and cellular telephones. A receiver circuit typically receives a radio frequency (RF) modulated signal from an antenna. The airways are continuously flooded with many distinct RF signals operating at different frequencies. The user, for example operating a cordless telephone, is only interested in a particular frequency, namely the channel frequency transmitted from the matching base unit. The receiver should as much as possible differentiate the desired RF signal and eliminate unwanted interfering RF signals, for example from a neighbor's cordless telephone, operating in an adjacent frequency channel.
The RF signals can range from power levels as low as -120 dBm to higher power levels around -10 dBm. A problem arises when the desired RF signal operates at a low power level in one frequency channel and the interfering RF signal in at least two adjacent channels operate at a higher power level. The receiver includes a mixer that downconverts the RF signal to an intermediate frequency (IF). During the mixing process, a number of inter-modulation products are generated involving the adjacent channel RF signals. In particular, the third order inter-modulation component of the adjacent channel RF signals can be translated to a frequency at or near the desired IF frequency and thereby interfere with the desired IF signal. If the adjacent channel RF signals inter-modulate directly onto or near the desired IF signal frequency, then the adjacent channels become indistinguishable from the desired channel. The distortion product can overwhelm the desired IF signal if the inter-modulation power is greater than the desired IF signal power. The interfering RF signal can overpower and prevent the receiver from discerning the desired RF signal.
In the prior art, the receiver includes an input low-noise amplifier (LNA) followed by a filter and the mixer for downconverting the RF signal to an IF signal according to a local oscillator (LO) signal. The receiver uses an automatic gain control (AGC) loop that senses the power levels of the incoming RF signals and adjusts the gain of the input LNA accordingly to control the dynamic range of the signal processed through the receiver. The AGC controls inter-modulation interference by reducing the gain in the receive signal path, typically by controlling the LNA, and thereby reducing the signal level of the adjacent channels going into the mixer.
One problem with the AGC approach is that reducing the gain of the input LNA adversely impacts the noise contribution of subsequent receiver stages and degrades the overall receiver noise figure, i.e. ratio of output SNR to input SNR. If the gain of the input LNA is high, then noise contribution from the other receiver components, e.g. filter and mixer, is non-dominant. That is, with high input LNA gain, the RF input signal is a sufficiently high level that any noise from the filter and mixer does not adversely affect the receiver sensitivity, i.e. minimum level RF signal above the noise floor that can be detected. However, when the gain of the input LNA is low, then the noise contributions from the filter and mixer becomes appreciable. The lower gain of the input LNA is especially problematic when the adjacent channel signal power is greater than the desired channel signal power. The desired channel becomes distorted with noise and the sensitivity of the receiver decreases. It is desirable to maintain the receiver sensitivity and the noise figure constant over a wide range of input signal levels.
Hence, a need exists to control the dynamic range of the receiver independent of the system noise figure and without degrading the receiver sensitivity.